I've also faced up to the same problem as the one you are describing and resetting the toolbox did the job for me.
That's why I did a bit of searching around the net for a few things to try and found these posts in the stackoverflow site:
according to which the problem probably consists in that you have your toolbox items in the on-disk cache or settings. It basically lists these steps:
1. Close Visual Studio
2. Ensure no instances of devenv is running.
3. Go to Users\\AppData\Local\Microsoft and look for Visual Studio temp files.
4. Delete the temp files.
5. Go to Users\\Documents\VisualStudio 2008\Settings and make backups of your settings files, then delete the originals.
6. Reopen VS.NET, you should be prompted to choose your developer settings.
and this one:
Hope these posts will help you to resolve your problem.
Regarding the 'huge number of "controls" with no icons in the list' , the problem is coming from the design assemblies, but we are currently working on some toolbox improvements and hopefully they will be available for our next official release.
the Telerik team